The idiom let sleeping dogs lie means not to stir up old conflicts or provoke an argument over unresolved issues. It is often better to agree to disagree and move forward rather than trigger hostilities over an ongoing difference of opinion. Let sleeping dogs lie refers to the instinctive behavior of a dog whenever it is awakened suddenly from a nap. Many dogs instinctively lash out at those who try to awaken them without reason, and these natural responses are often quite painful.
One example of the idiom in action would be a heated discussion between a man and wife over an unpaid bill. While the issue itself may be relatively minor, it could bring up painful memories of a previous fight over household finances. Instead of provoking or escalating the argument, both might agree to let sleeping dogs lie and not remind each other of the previous fight unnecessarily. Dredging up a provocative or painful issue from the past would not help to resolve the current situation, so both sides agree not to discuss it.
Unresolved grudges or long-standing feuds in the business world can also benefit from this proverb. A project manager may have had an unpleasant working relationship with a subordinate in the past, but a new project requires complete team cooperation. Rather than sabotage group unity, the project manager and his co-worker may agree to let sleeping dogs lie where their personal disagreements are concerned. If the past issues are largely resolved and have little bearing on the present situation, then it is better for everyone that they remain dormant.
Although the Old Testament book of Proverbs makes references to the foolish mishandling of dogs, many sources point to the English author Chaucer as the originator of the idiom in print. Other authors have since used variations on Chaucer's theme to imply the inherent dangers of awakening sleeping dogs. The modern form of the idiom is more passive in nature, suggesting that sleeping dogs, especially large and volatile breeds, should remain undisturbed. In Chaucer's time, the proverb warned against deliberately approaching a sleeping dog and waking it up forcefully. Anyone who chooses to wake up a sleeping guard dog generally gets the punishment he or she deserves.